The invention departs from the state of the art, as is known for example with the Grundfos pump construction series of the type CR and CRE. With regard to these pumps, it is the case of multi-stage, vertically operated centrifugal pumps with a foot part and with a head part, between which at least two pump stages are arranged, said pump stages each comprising an impeller and a housing surrounding this. The suction connection and the delivery connection of the pump are formed in the foot part. The fluid to be delivered gets through a suction branch, which is likewise arranged in the foot part of the pump, firstly into the first pump stage and further up to the last pump stage, where it is led back into the foot part again via an annular channel formed between the outer casing and the housings, and there is led out of the pump via a delivery branch. The drive shaft of the pump which connects the impellers is led out through the head part in a sealed manner, where a motor bracket connects, said motor bracket being provided for receiving an electrical drive motor and in which the pump-side shaft is coupled in movement to the motor-side shaft. With regard to the above mentioned pump construction series, it is the case of pumps which are vertically operated, which means to say are operated in a manner standing on the foot part with a vertical impeller axis, but for the present invention it is basically of no concern as to whether the pump is operated vertically or in any other position.
With these known pumps, the head part and the foot part are typically formed from a metallic cast material, whereas the outer casing consists of sheet metal shaped into a cylinder. The outer casing and the pump stages are clamped between the head part and foot part and are held by four clamping bolts which are led through bores in corners of the head part and foot part, said corners laterally projecting beyond the cylinder body.
The pumps of the previously mentioned construction series have proven their worth and are on the market in numerous different power and housing embodiments.
Stress conditions within the pump can undesirably change, if such pumps are operated at high temperature differences between the medium to be delivered and the environment, since the clamping bolts (tension bolts) undergo a different thermal expansion than the housings of the pump stages. Objects or flying parts can jam or at least settle in this region, on account of the clamping bolts which are arranged at a small distance to the housing casing, and this is not desirable. These clamping bolts also upset the overall optical impression of the pump, which is otherwise impressed by the simple cylinder casing which is mostly formed from stainless steel. A cladding of the clamping bolts however is mostly cumbersome and expensive.
A multi-stage pump from U.S. Pat. No. 2,957,426 is counted as belong to the state of the art and with this, the pump stages can be selectively connected in parallel or in series. The individual pump stages are connected there via stud bolts which partly run through the pressure channel and connect the pump stages to an end-part of the pump housing peripherally surrounding all pump stages and at a distance at a face side.